Newly understood instrumentation readings returned in the 1990s from NASA’s Galileo spacecraft in orbit of Jupiter have revealed that the craft not only encountered but also flew through ejecta plumes from Europa’s subterranean ocean. The announcement this week was the culmination of years of work, study, and analysis, and paves the way for interesting discoveries for the upcoming Europa Clipper mission which already includes instruments to study such plumes.

Data collected by NASA’s Galileo spacecraft in 1997 were put through new and advanced computer models to untangle a mystery first presented to the Galileo science teams – a brief, localized bend in the magnetic field that had gone unexplained until now. Previous ultraviolet investigations of Europa conducted in 2012 by the Hubble Space Telescope suggested the presence of water ejecta plumes, but definitive evidence for such characteristics at Europa remained elusive.

This new analysis of Galileo data from 1997 is considered strong, corroborating support for water ejecta plumes.

The craft remained in orbit of Jupiter for 7 years 9 months and 13 days before being purposefully deorbited on 21 September 2003, crashing into Jupiter’s atmosphere at 173,700 km/h (108,000 mph) after 35 orbits of the giant planet. Galileo was purposefully destroyed to ensure that the craft could not accidentally contaminate any potentially life harboring moons of Jupiter, notably Europa.

Now, old information regarding a Europa flyby in 1997 has yielded exciting information about the ice-covered moon and adds another item of discovery to Galileo’s catalog. The new research was led by Xianzhe Jia, a space physicist at the University of Michigan in Ann Arbor. “The data were there, but we needed sophisticated modeling to make sense of the observation,” Jia said.

Jia’s team was inspired to dive back into the Galileo data by Melissa McGrath of the SETI Institute in Mountain View, California. “One of the locations she mentioned [from her Hubble observations of Europa] rang a bell. Galileo actually did a flyby of that location, and it was the closest [flyby] we ever had. We realized we had to go back,” Jia said. “We needed to see whether there was anything in the data that could tell us whether or not there was a plume.”

At the time of the 1997 flyby, about 124 miles (200 kilometers) above Europa’s surface, the Galileo team didn’t suspect the spacecraft might be grazing a plume erupting from the icy moon. Now, Jia and his team believe the craft’s path past Europa was fortuitous.

When the team examined the information gathered during that flyby 21 years ago, high-resolution magnetometer data showed something strange. Drawing on what scientists learned from exploring plumes on Saturn’s moon Enceladus – that material in plumes becomes ionized and leaves a characteristic blip in the magnetic field – they knew what to look for. And there it was on Europa: a brief, localized bend in the magnetic field that had never been explained before.

One of Galileo’s instruments was a Plasma Wave Spectrometer, used to measure plasma waves caused by charged particles in gases around Europa’s atmosphere. Jia’s team pulled that data as well, and it also appeared to back the theory of a plume. But numbers alone weren’t enough to confirm that Galileo had indeed flown through an ejecta plume from Europa. So Jia layered the magnetometry and plasma wave signatures into new 3D modeling developed by his team at the University of Michigan, which simulated the interactions of plasma with solar system bodies. The final needed pieces of information came from the 2012 data from Hubble that suggested dimensions of potential plumes.

The result that emerged, with a simulated plume, was a match to the magnetic field and plasma signatures the team pulled from the Galileo data. “There now seem to be too many lines of evidence to dismiss plumes at Europa,” said Robert Pappalardo, Europa Clipper project scientist at NASA’s Jet Propulsion Laboratory. “This result makes the plumes seem to be much more real and, for me, is a tipping point. These are no longer uncertain blips on a faraway image.”

The findings are good news for the Europa Clipper mission, which may launch as early as June 2022. From its orbit of Jupiter, Europa Clipper will sail close by the icy moon in rapid, repeated, low-altitude flybys. If plumes are indeed spewing vapor from Europa’s ocean or subsurface lakes, Europa Clipper could sample the frozen liquid and dust particles.

The mission team is gearing up to look at potential orbital paths, and this new research will play into those discussions. “If plumes exist, and we can directly sample what’s coming from the interior of Europa, then we can more easily get at whether Europa has the ingredients for life,” Pappalardo said. “That’s what the mission is after. That’s the big picture.”

Europa Clipper’s launch on SLS is mandated by the U.S. Congress via federal law. The Trump Administration, in its FY 2018 appropriations request, sought to eliminate the SLS mandate from Europa Clipper’s mission; however, when Congress passed a two your spending agreement earlier this year, they maintained the SLS launch vehicle mandate for the mission.

From a congressional standpoint, Europa Clipper’s mandate to launch on SLS is driven by political security for the SLS rocket, granting it a high priority science mission in its early years of operation. However, from a scientific reality, SLS is the only launch vehicle capable of launching Europa Clipper into a direct insertion trajectory to Jupiter, foregoing multiple gravity assist flybys of Earth and Venus and allowing the spacecraft to reach Jupiter in a much shorter time period than would otherwise be achievable on an Atlas V or Falcon Heavy – which would both have to rely on gravity assist trajectories to get Europa Clipper to Jupiter.

But regardless of which vehicle Europa Clipper launches on, the mission is already designed to take maximum advantage of the newly discovered evidence for plumes at Europa from the Galileo mission.

In particular, the Europa Thermal Emission Imaging System, E-THEMIS, will provide high spatial resolution, multispectral imaging of Europa in the near- and far-infrared bands and will be able to detect active sites on Europa’s surface – such as potential vents erupting plumes of water from Europa’s subterranean ocean and lakes into space.

Moreover, the Plasma Instrument for Magnetic Sounding, PIMS, will seek to unlock the mechanisms by which weathering and releasing of material from Europa’s surface into the atmosphere and its ionosphere occurs while the Mass Spectrometer for Planetary Exploration, MASPEX, determines the composition of the surface and subsurface ocean by measuring Europa’s tenuous atmosphere and any materials ejected from the moon out into space.

While these instruments and Europa Clipper overall were designed to be the first instruments and spacecraft to definitively detect whether or not Europa was ejecting plumes into space, the instruments are incredibly well-suited to now explore the known plumes Europa hurls out into space from its subterranean ocean.

During a science discussion and media event yesterday, scientists working on the Europa Clipper mission confirmed that none of the instruments aboard the spacecraft will be changed based on the newly discovered information from the Galileo archive. However, the Galileo data will go a long way towards allowing scientist to finalize and tweak the flyby altitudes, locations, and trajectories Europa Clipper will follow during its in-situ observations of the ice-covered moon.